docs: def reduce(self, sitecol):
          """
          :returns: a reduced AssetCollection on the given sitecol
          """
          ok_indices = numpy.sum(
              [self.array['site_id'] == sid for sid in sitecol.sids],
              axis=0, dtype=bool)
          new = object.__new__(self.__class__)
          vars(new).update(vars(self))
          new.array = self.array[ok_indices]
          new.array['ordinal'] = numpy.arange(len(new.array))
          new.asset_refs = self.asset_refs[ok_indices]
          return new